The invention relates to a coupling circuit of the type comprising two inputs and one output, by means of which an output signal is obtained, which is the sum of the two input signals; the invention further relates to a coupling and mixing circuit, by means of which an output signal is obtained which is the mixed product of the two input signals.
The invention applies to transmissions of signals, for example, by means of artificial satellites, and in particular to circuits for receiving and processing these signals. The circuit in accordance with the invention serves for the transposition of frequencies to be effected between the emission frequency of the signals (for example, for broadcasting systems using artificial satellites, this frequency may lie between 11.5 and 12.5 GHz) and an intermediate frequency (for example, of 1 GHz, for the same systems).
The choice of the frequencies for the broadcasting systems (and more generally even telecommunication systems) is determined by the environment in which these signals will be emitted and by the power available in the transmitter station. The quality of a signal is only acceptable when the ratio between the level of the useful signal and the disturbing noise level is sufficiently large. A part of this noise is received by the receiving antenna, whilst another part is produced in the receiver. The noise received by the antenna can be reduced only by a suitable choice of the frequencies.
The operating frequencies are therefore chosen in the radio-frequency ranges roughly between 0.5 and 50 GHz. The frequency bands recently allocated by the CCIR for broadcasting through artificial satellites lie between 11.7 and 12.5 GHz.
The high-frequency signal emitted by the satellite is received on the earth by a communal or individual station and converted into a signal of lower frequency in order that it may be accepted by a standard television apparatus. In fact this frequency change takes place in several steps: A first frequency change from the emission frequency (12 GHz) to an intermediate frequency (1 GHz), while other frequency changes take place later on to the frequency of approximately 130 MHz, at which the frequency demodulation and the amplitude modulation of a carrier frequency to be applied to a standard television receiver are effected.
According to the prior art, the frequency changes were effected by means of a Schottky diode of high cut-off frequency or, for example, a field effect transistor followed by a suitable filter for selecting the desired frequency from the various frequencies present at the output. Such a construction included transmission lines, for example, of the microstrip type, and coupling means for delivering the signals to be mixed at the input of the said diode.
Such a construction has the great disadvantage that it is of too large a size.